1. Field of the Invention
This invention relates to an improvement in a fluid coking process.
2. Description of the Prior Art
Fluid coking is a well-know process which may be carried out with or without recycle of the heavier portion of the fluid coking zone effluent. As is well known in the art, the fluid coking process, as shown, for example, in U.S. Pat. No. 2,881,130, which is hereby incorporated by reference, uses a fluid coking vessel and an external heating vessel. A fluid bed of solids, preferably coke particles produced by the process, having a size in the range from about 40 to about 1000 microns, is maintained in the coking zone by the upward passage of fluidizing gas, usually steam, injected at a superficial velocity, typically between 0.3 and 5 feet per second. The temperature in the fluid coking bed is maintained in the range of about 850.degree. to 1400.degree. F., preferably between 900.degree. and 1200.degree. F. by circulating solids (coke) to the heating vessel and back. The heavy oil to be converted is injected into the fluid bed and upon contact with the hot solids undergoes pyrolysis evolving lighter hydrocarbon products in the vapor phase, including normally liquid hydrocarbons, and depositing a carbonaceous residue (coke) on the solids. The turbulence of the fluid bed normally results in substantially isothermal reaction conditions and thorough and rapid distribution of the heavy injected oil. Product vapors, after removal of entrained solids, are withdrawn overhead from the coking zone and sent to a scrubber and fractionator for cooling and separation. The end boiling point of distillate fractions obtained from the process is usually about 900.degree. F. to about 1050.degree. F. and the remaining heavy ends are typically recycled to extinction.
U.S. Pat. No. 4,426,277 discloses a fluid coking process in which the feed is first passed into a dense phase zone and then into a transfer line zone. A third coking zone is operated at a higher temperature than the first two zones.
U.S. Pat. No. 2,879,221 discloses a two-stage coking process in which the second stage is operated at a higher temperature than the first stage.
U.S. Pat. No. 2,853,434 discloses a two-stage coking process in a unitary vessel. Each stage contains a fluidized bed.
An improved low severity fluid coking process has now been found in which coke and gas yields may be decreased relative to conventional fluid coking while increasing the yield of liquid products.